This invention relates to submersible propulsor units, and is specifically concerned with a high power, low noise two-stage submersible propulsor unit that is particularly adapted to provide secondary propulsion for submarines and other special vehicles.
Electric motor type propulsor units are known in the prior art. While such propulsors may be used for surface vessels, they find their primary application as secondary drive units for submarines where reliability, low noise emissions, and shock resistance are at a premium. In the prior art, such propulsor units have typically comprised a "canned" or wet-winding type electric motor having an output shaft that is connected to a propeller. Unfortunately, the fact that the canned motor is disposed either directly in front of or behind the flow of water generated by the propeller creates an obstruction to fluid flow that results in unwanted noise, and reduces the effective thrust that can be generated by these units. To compensate for the thrust losses caused by this blockage, higher speed and smaller diameter motors may be used. However, the high shaft speed of these motors results in a high propeller cavitation, which in turn generates an even higher level of unwanted noise.
To overcome these shortcomings, the Westinghouse Electric Corporation developed an integral motor propulsor unit that is disclosed and claimed in U.S. Pat. No. 4,831,297. This particular propulsor unit resembles a jet engine in structure and generally comprises a cylindrical shroud having a water inlet and a water outlet, a propeller having a hub rotatably mounted within the shroud on a shaft that is concentrically mounted within the shroud by a plurality of support vanes, and an electric motor for driving the propeller that includes an annular rotor mounted around the periphery of the propeller blades, and a stator that is integrated within the shroud of the unit. The advanced design of this particular prior art propulsor unit substantially increases the thrust output for a propulsor for a given weight and size while at the same time reducing the amount of noise generated due to the largely unencumbered flow of water that the propeller of the device can force through the fluid-dynamically shaped shroud. The quietness of the unit is further improved due to the noise-blocking characteristics of the shroud.
While the aforementioned integral motor propulsor unit provides a substantial advance in the art, the applicants have noted a number of limitations associated with the design of this unit which impair its usefulness in submarine applications. For example, if the thrust output of such a prior unit could be substantially increased, and noise emissions reduced, the uses of the unit could be expanded beyond emergency backup applications and maneuvering applications. Such an increased thrust capacity would allow it to become a truly redundant thrust system should the need arise to take the primary thrust system of the submarine off-line. Of course, such prior art propulsor units could be upscaled to produce more power. However, for certain submarine applications, there are limitations with respect to the width of the propulsor unit which do not allow such an overall upscaling of the device to solve the problem of the need for increased thrust. Specifically, as the width of the propulsor unit increases, the unit as a whole exposes more and more area to fore and aft shock waves that military submarines might be exposed to during combat. Still another limitation resides in the fact that the initial flow of sea water into the propeller within the shroud must be substantially if not completely unencumbered in order to avoid the generation of an unacceptably large amount of unwanted noise caused by cavitation in the inflowing water. Such cavitation may be induced, for example, by upstream support vanes which secure the propeller shaft to the inner diameter of the shroud, or by bearing arrangements which require an enlargement in the upstream end of the propeller shaft. Still another limitation associated with such prior art propulsor units resides in the design of the water lubricated thrust and radial bearings which periodically need to be maintained or replaced. To perform such maintenance or replacement, the unit must be dry docked, and the mechanical arrangement of these bearing components necessitates an almost complete disassembly of the propulsor unit. Finally, the applicants have further observed that the design of the electric motors used in such prior art propulsor units necessitates a very close spacing between the outer diameter of the rotor and the inner diameter of the stator if the electromagnetic coupling between the rotor and the stator is to be effectively implemented. However, such close spacing not only creates drag forces from the thin film of water that is disposed between the stator and the rotor; it also generates additional unwanted noises by increasing the magnitude of harmonic currents flowing through the rotor (which are always present to some degree due to the dissymmetries in the magnetic fields generated by the stator) which in turn causes the rotor to vibrate. The close spacing required between the inner diameter of the stator and the outer diameter of the rotor also creates an unwanted area of vulnerability in the propulsor unit should it be subjected to a high level of mechanical shock, or should salt water debris collect between the stator and the rotor.
Clearly, there is a need for an improved, integral motor-type propulsor unit for use in submarines or other water vessels which maintains all the advantages associated with the latest prior art propulsor unit, but which is capable of generating larger amounts of thrust with a mechanism which does not exceed the maximum width limitations associated with submarine applications. Ideally, such a propulsor unit would have a lower noise signature than prior art units, and would incorporate a design which does not necessitate such close spacing between the rotor and stator in order to decrease the vulnerability of the unit in this region to mechanical shock, or the collection of sea water debris. The bearing assemblies should be easily accessible in the event that a repair or a maintenance operation were necessary without the need for dry docking the unit, or for a large amount of disassembly of the unit. Finally, it would be desirable if the unit were capable of more efficiently generating usable thrust, and were more reliable than prior art propulsor units.